The present disclosure generally relates to vibrating structure gyroscopes and more specifically to Microelectromechanical System (MEMS) based vibrating structure gyroscopes. Vibrating structure gyroscopes utilize solid-state resonators, of different shapes, to measure orientation or rotation rate based on the principle that a vibrating object tends to continue vibrating (i.e., oscillate) in a fixed orientation in space as its support rotates, and any vibrational deviation of the object can be used to derive a change in direction. Vibrating structure gyroscopes may be manufactured with MEMS based technology. For example, vibrating structure gyroscopes may be fabricated on silicon or glass wafers using a sequence of steps including photolithography, etching and deposition, or any other MEMS based technology.
Vibrational deviations in a resonator of a MEMS based gyroscope may be caused by a Coriolis force. For example, a mass moving at a given velocity will experience Coriolis acceleration when the mass is also rotated with an angular velocity. The Coriolis acceleration is perpendicular to the velocity and the angular velocity. The Coriolis acceleration vector is given by ac=−2(v×Ω), where v is the velocity vector and Ω is the angular velocity vector. Coriolis acceleration is thus indicative of the angular velocity of rotation.
Many MEMS based gyroscopes are configured to operate in a rate mode. In a rate mode of operation, vibration of one axis (i.e., a drive axis) of a MEMS based gyroscope is driven at a fixed amplitude in a closed loop while Coriolis-induced motion is read out on the other axis (i.e., a sense axis). In such a rate mode, the amplitude of the Coriolis-induced motion read out on the sense axis is indicative of a rate of angular movement of the gyroscope. Rate mode operated gyroscopes are limited in that the Coriolis-induced motion measurements are limited by the dynamic range of the open-loop sense axis. For fast movements of the gyroscope, the open-loop sense axis may not be able to “keep up” with the movement of the gyroscope. In addition, spring non-linearities at high rates of rotation may cause errors. Some MEMS based gyroscopes operated in rate mode attempt to avoid these problems by also operating the sense axis in a closed loop and monitoring the level of force required to maintain the amplitude of the sense axis at a fixed level. However, such gyroscopes are limited by the closed sense loop bandwidth and the maximum force capable of being exerted by the rebalance.
One example of a MEMS based gyroscope configured to operate in a rate mode is a Tuning Fork (TF) gyroscope. A Tuning Fork gyroscope includes a pair of relatively large lumped-element proof masses that are driven to oscillate, in an in-plane axis, with equal amplitude but in opposite directions. When a TF gyroscope is rotated, the Coriolis force creates an orthogonal vibration (i.e., an out-of-plane vibration) in the proof masses that can be sensed by a variety of mechanisms. By monitoring out-of-plane vibrations of the proof masses, the rate of rotation of the TF gyroscope can be determined.
Another mode of operation for MEMS based gyroscopes is a whole angle mode (otherwise known as an integrating or rate integrating mode). In a whole angle mode of operation, two axes, having identical frequency and damping, are coupled by Coriolis motion. The axes are driven such that the total vibrational amplitude of the two axes is sustained, but the distribution of energy between the two axes is allowed to change freely. Accordingly, a Coriolis force causes energy to be transferred from one axis to the other as the gyroscope rotates. By measuring the distribution of energy between the axes, an angle of rotation (with respect to a starting angle) can be read out. As energy can freely transfer from one axis to the other in a MEMS based gyroscope operating in whole angle mode, there is no limit on the rate at which the axes can transfer motion. As such, whole angle operating gyroscopes avoid the dynamic range issues discussed above with regard to rate mode operating gyroscopes and typically provide a higher level of performance and higher bias stability.
One important requirement of a whole angle operating MEMS based gyroscope is that the two modes be identical (i.e., degenerate) with regard to frequency and damping. If the frequencies differ substantially, a Coriolis force caused by rotation of the gyroscope will not be sufficient to transfer energy from one mode to the other and the vibration will stay “locked” to a single axis. This will interfere with the free transfer of motion between modes and the free precession of the mode shape of the gyroscope. A whole angle operating MEMS based gyroscopes must therefore be designed and fabricated with exceptional symmetry and with mode structures that are insensitive to expected fabrication variations. In addition, it is also typically desired for whole angle operating MEMS based gyroscopes to provide low damping (i.e., long ring down time) and matched damping for principle axes. This is because low overall damping correlates to low damping differences between the two axes and on-axis damping may result in gyroscope bias when drive forcers are misaligned. A mismatch (or mismatch drift) may result in a bias (or bias drift).
Traditional whole angle MEMS based gyroscopes include an axially or cylindrically symmetric and continuous structure that is driven to excite two vibratory modes of the structure (i.e., an n=2 vibratory mode where two points of the ring are moving away from the center of the ring while two other points of the ring are moving toward the center of the ring). Rotation of the gyroscope results in a Coriolis force that causes movement (i.e., either inward or outward motion) of other points of the symmetric structure. By monitoring the movement of the symmetric structure in two in-plane axes, the angle of rotation of the gyroscope can be determined.
One common example of a whole angle operating gyroscope is a Hemispherical Resonator Gyroscope (HRG) (otherwise known as a wine-glass gyroscope). An HRG includes a thin hemispherical shell, anchored by a stem. The shell is driven to a flexural resonance and a gyroscopic effect is obtained from the inertial property of resulting flexural standing waves. An HRG is typically reliable and accurate; however, they are also typically large and costly.
Another example of a whole angle operating gyroscope is a ring gyroscope. Ring gyroscopes include axially symmetric and continuous rings that are driven in an n=2 vibratory mode, as discussed above. The movement of the ring is monitored to determine an angle of rotation of the ring gyroscope. The performance of such ring gyroscopes is limited in that due to the limited mass of the rings, the sensitivity of the gyroscope is relatively low and the bias instability is relatively high.